1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a recording head for ejecting liquid droplets.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that uses a recording head (liquid-droplet ejection head) for ejecting droplets of ink.
Such inkjet-type image forming apparatuses fall into two main types: a serial-type image forming apparatus that forms an image by ejecting droplets from the recording head while moving a carriage mounting the recording head in a main scanning direction, and a line-head-type image forming apparatus that forms an image by ejecting droplets from a linear-shaped recording head held stationary in the image forming apparatus.
Such an inkjet-type image forming apparatus may time-serially generate multiple driving pulses (ejection pulses) for ejecting droplets within one driving cycle to output a common driving waveform. For example, to form a relatively large dot, two or more driving pulses are selected to eject multiple droplets. Then, multiple droplets merge during flying and land on, e.g., a sheet of recording media to form the large dot on the sheet, thus allowing dots of different droplet sizes to be formed on the sheet. In addition, the image forming apparatus may incorporate a non-ejection pulse into the common driving waveform to drive the recording head without ejecting droplets. By selecting the non-ejection pulse, minute driving of the recording head can be performed to stably eject droplets.
To pressurize liquid in a liquid chamber to eject droplets of the liquid, for example, a driving pulse of a conventional driving waveform contracts the liquid chamber from an expanded state to eject liquid droplets, temporarily retains a contracted state, of the liquid chamber, further contracts the liquid chamber, and expands the liquid chamber.
In this regard, when a liquid droplet is ejected from a nozzle of the liquid ejection head, a droplet tail portion (hereinafter, “satellite”) leading from the liquid droplet to a meniscus of liquid in the nozzle is created. The liquid droplet separates from the satellite and flies toward the sheet. The higher the viscosity of the liquid ejected from the nozzle, the longer the satellite. When the satellite separates from the meniscus of liquid in the nozzle, the satellite flies as a satellite droplet (by contrast, the above-described precedent flying liquid droplet is referred to as “main droplet”).
To increase the print speed or print gap (between the nozzle and the recording media) in the image forming apparatus, it is preferable to shorten the length of satellites in ejecting the main droplets, minimize occurrences of satellite droplets, or prevent satellite droplets from landing on positions differing from the main droplets. In particular, in a case where multiple recording heads are arranged, if satellite droplets occur at different states between the recording heads, the color tone (e.g., brightness) of a resultant image may vary, thus affecting image quality. In addition, such different states of satellite droplets may result in, e.g., a reduced accuracy in reading a resultant bar code or a reduced image quality (e.g., blur) of characters.